Light fastness standards

Pugh, S. L., Guthrie, J. T., The Development of Light Fastness Testing and Light Fastness Standards, Rev. Prog. Color. Relat. Top. 2001, 31, 42 56. 

Standards exist for determining the light fastness of colored pigmented plastics in daylight and in xenon arc light, i.e. in accelerated exposure equipment, and for determining the weathering fastness. 

Copolymers of itaconic esters with butadiene have not yet been used technically. On the other hand, acrylonitrile containing copolymers with other components have been studied from several points of view. Standard Oil Co. has claimed a terpolymer of isobutylene, butadiene, and acrylonitrile, and BASF a similar product of butadiene, acrylonitrile, and styrene. The films from these combinations are said to have high flexibility and cold resistance. However, all butadiene containing copolymers are not light fast. Copolymers of butadiene, acrylonitrile, and unsaturated dicarboxylic esters are suggested for plasticizing PVC, but they must be thermally degraded before they are combined with the polymer. 

Dyed samples were also evaluated for resistance to color changes by rubbing (crockfastness), exposure to light (lightfastness) and exposure to water (wet-fastness). Standard test methods recommended by the American Association of Textile Chemists and Colorists were used in the fastness evaluations. 

A simple and usable method has thus been found for determining the light fastness of dyed materials. There was still a need for a standard method for the evaluation of dyes. As is well known, a dye fades more rapidly in light tints than in the deeper tints, and different fastness ratings are obtained for one and the same dye depending on the depth of tint. 

However, serious defects were soon apparent even in this system. After the six standard scales were prepared, requiring 33 dyes, the process of arriving at a reliable estimation of the degree of light fastness was very complicated and difficult because of the variations in color in the standards. Also, there was the difficulty that there was no uniformity of gradation in the standard dyeings with different textile fibers,... 

The introduction of these standards simplified matters a great deal. By their use, it became possible to compare dyes of any depth or tint and, of special importance, on all the various fibers. Thus, this set of blue standards has become an absolute standard for the determination of light fastness and it has become possible to assign a numerical value for light fastness to every dye used in practice. 

The relationship, between relative humidity and light fastness is also reviewed by McLaren (J.S.D.C., 1963, 79, 618). Specimens of red cotton dyed with an azoic dye, known to be moisture sensitive with regard to light fastness, were exposed, together with standards in constant-humidity sealed vessels. The results of these experiments are shown graphically in Fig. 25.7. It will be noticed that on the light fastness ordinate, 6-7 appears where 7 would normally be expected. This is because up to standard 6, each grading is twice as fast as the one below, but standard 7 is four times as fast as 6. 

Comparison of the (fyeing behavior of standard polyesters with that of copolyesters confirms that block-copolyesters containing polyalkylene glycols and PET modified by alitdiatic or aromatic compounds show a markedly better dye uptake, uniformity, and light fastness than standard polyesters, especially the copolyesters of the latter type. 

ASTM D 3424 [427] Standard Practice for Evaluating the Relative Light fastness and Weatherability of Printed Matter... 

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